The overall objective of the proposed research is to elucidate the molecular mechanisms and the cellular consequences of normal and abnormal DNA repair in human cells. Cell lines from normal individuals, patients with xeroderma pigmentosum (XP), and patients with Cockayne's syndrome (CS) will be studied. XP and CS are cutaneous photosensitivity diseases in which patients have genetic defects in the ability to repair ultraviolet light-induced DNA damage in epidermal and other cells. XP patients are extremely susceptible to sunlight-induced skin cancer; their cell lines, thus comprise an excellent system with which to study the relationships between DNA repair, mutagenesis, and carcinogenesis. Our proposed studies will: 1) expand the molecular characterization of cell lines from these repair-deficient individuals by using agarose gel electrophoresis to measure the removal of pyrimidine dimers from DNA, and by using alkaline sucrose gradient analysis and the alkaline elution assay of Kohn to measure post-replication repair of UV damage, and removal of both DNA interstrand crosslinks and DNA-protein crosslinks; 2) generate human lines with new repair defects by mutagenizing human cells with ethylmethane sulfonate and selecting for cells with altered ultraviolet sensitivity; 3) measure and compare ultraviolet-mutability at the hpt locus and the locus for ouabain sensitivity in lymphoblasts from normal and repair-deficient individuals; 4) assess the role of the cell cycle phase in ultraviolet cytotoxicity and mutagenesis by measuring both phenomena in cells at various times after release from blockade at the Gl/S boundary; 5) determine the ultraviolet action spectrum for cytotoxicity and mutagenesis in normal and repair-deficient cells by utilizing light sources with different spectral outputs; and 6) determine whether, in human cells, the activation of "inducible" DNA repair systems can alter subsequent levels of ultraviolet-related cytotoxicity or mutagenesis. Together, these interrelated studies are designed to increase our understanding of the relationship between human DNA repair and environmentally-induced disease of the skin and other organs.